The present invention relates to an optical isolator designed to reduce the displacement (walk-off) between the ordinary and extraordinary rays transmitted through the isolator.
Reflections in optical systems often generate noise and optical feedback which may degrade the performance of various system components, particularly signal sources such as semiconductor lasers and fiber amplifiers. Therefore, the ability to optically isolate lasers and other sensitive components from these reflections is often critical to the performance of the system. In general, a conventional optical isolator comprises a non-reciprocal rotator (e.g., 45.degree. Faraday rotator encased within a bias magnet) disposed between a pair of polarization selectors (e.g., linear polarizers, birefringent plates, or birefringent wedges). Input signals passing through the isolator in the transmitting, forward direction are essentially fully coupled through the polarization selector and Faraday rotator to the output device (e.g., fiber). However, the polarization of reflected signals passing through the isolator in the reflecting, reverse direction is rotated such that essentially none of the reflections are coupled back into the signal source.
In many important system applications, in particular those where the polarization of the signals cannot be fully controlled throughout the system (e.g., in most fiber optic systems), it is important that the isolator operate effectively regardless of the polarization state of the signal.
A common isolator design which addresses this problem includes a Faraday rotator slab disposed between a pair of birefringent wedges. See, for example, W. Emkey et al., Proc. OFC, Abstract THF2 (1989). The input signal is incident on the oblique surface of the input wedge at essentially the wedge angle. (For comparison purposes, we note that this incident angle is measured below the normal to the oblique surface; that is, from the normal toward the thicker end of the wedge.)
Although this design may be polarization independent under certain conditions, it does exhibit a phenomenon called relative walk-off. That is, the input signal splits into ordinary and extraordinary rays which arc displaced from one another at the isolator output. When the output is lensed into an aperture (e.g., a fiber core), the power coupled into the fiber from each ray can be different, and the total coupled power is less than that possible if the relative walk-off did not occur.
Thus, a need remains in the optical isolator art for a technique which reduces walk-off between the ordinary and extraordinary rays.